The medical examiner's findings were filed in May, according to an article in the January issue of Spin about the convergence of comedy and rock 'n' roll.
Hedberg, 37, died in a hotel room in Livingston, N.J., while on a club tour performing standup. The comedian, who was born with a heart defect, was initially believed to have died from heart failure.
He had struggled with drugs and alcohol, and took a hiatus from performing for several months after a May 2003 arrest in Austin, Texas, for felony possession of heroin.
In the Spin article, Maureen Taran, a comedy manager who knew Hedberg, is quoted as saying that life on the road is difficult for a struggling comedian.
"It's a very lonely existence," Taran says. "If you aren't born with manic-depression, you will have it after being in this business."
But TV-series fame eluded him because his unique style of mumbled one-liners didn't lend itself to the sitcom format.
Born in St. Paul, Minn., Hedberg rose through the ranks at Minneapolis' Acme Comedy Co. and caught his big break through a Comedy Central special.
Hedberg had two popular comedy CDs, "Strategic Grill Locations" and "Mitch All Together."